Optical signal processing and the long-haul transmission of optical data through optical fiber waveguides are becoming more widespread with the continuing technical evolution and economic viability of fiber-optic communications and sensor systems. The need for equipment such as optical regenerator/repeaters, which operate on the basis of optical to electrical signal conversion, electrical signal amplification, which then modulates the laser transmitter for further optical transmission, represents an impediment to full realization of the advantages of fiber-optic transmission systems. Similarly, in the field of fiber-optic data bus networks, optical amps or attenuators are needed to match the signal levels to requirements of the optical circuit or particular circuit components, which results in a number of impediments. The full realization of the advantages of optical transmission networks is not achieved, since the input energy density requirements of each of the elements in the optical circuit may vary. Presently, there is no convenient way to transition energy density at discrete places in an optical circuit to the precise level needed to induce, for example, the nonlinear optical effects needed for proper device functioning.
Relatively high intensity optical pulses transmitted in single-mode optical fibers have been observed to experience nonlinear effects during transmission through the fibers. These nonlinear effects, that is, stimulated Raman scattering, stimulated Brillouin scattering, self-phase modulation, intensity-dependent rotation of the linear polarization state, and stimulated four-photon mixing have all been studied theoretically, and to one extent or another, have been demonstrated experimentally in the laboratory. Furthermore, certain thresholds for the onset of these effects in single-mode fibers have been established.
The need for the incorporation of devices based upon nonlinear optical propagation effects into practical optical transmission links is just beginning to be recognized. Fiber-optic waveguides are becoming increasingly useful in the laboratory as a nonlinear medium for optical amplification and optical signal processing. The small diameter of the fiber core and the long interaction length are conducive to developing nonlinear effects. Hence, all-optical devices employing fibers have been demonstrated in the lab to obtain such functions as: direct optical amplification, optical gating or switching, optical pulse shaping, short pulse generation or pulse compression, dispersion compensation or the generation of soliton pulses.
An example of a device for optical gating using single-mode birefringent fiber has been demonstrated in the laboratory (see "Fiber-Optic Logic Gate" by K. Kitayama et al., Appl. Phys. Lett., Vol. 46, No. 4, (1985), pp. 317-319). The operation of the device is based upon the intensity dependent polarization rotation in birefringent fibers.
Another nonlinear optical device is the Kerr shutter described in the article by K. Kitayama et al., "Optical Sampling Using An All-Fiber Optical Kerr Shutter", Appl. Phys. Lett., Vol. 46, No. 7, (1985), pp. 623-625. In this case, energy from the pump light is injected into a highly birefringent fiber which results in a Kerr induced phase shift in this polarization preserving fiber thereby acting as the trigger for the shutter. Subsequent optical sampling and light modulation can be utilized in, for example, optical logic gates.
A laboratory demonstration of a nonlinear coupler switch capable of substantially complete all-optical switching at sub-picosecond rates has been reported ("Ultrafast All-Optical Switching In A Dual-Core Fiber Nonlinear Coupler," S. R. Friberg et al., Appl. Phys. Lett., Vol. 51, No. 15, (1987), pp. 1135-1137). By embedding two fibers parallel and adjacent to each other in a material having a large nonlinear coefficient (intensity dependent index of refraction), a change in input intensity can cause light to be switched from one waveguide to the other at the output of the device. The critical switching power for the demonstrated coupler is 850 watts. Switching speeds are extremely high but "will probably be limited by pulse broadening due to dispersion, rather than by the speed of the nonlinearity."
In any of the envisioned applications, the ability to obtain an optimum (relatively high) energy intensity inside the core of an optical fiber cannot presently be readily established. The peak powers needed are typically 100-1000 watts. Most of the obstacles preventing practical device conceptions are related to the high peak powers needed in the nonlinear optical devices. Since the nonlinear effects are directly related to the power density versus peak power, the reduction in the required levels of peak power, while retaining the required threshold values of intensity, can be obtained in two primary ways: (1) by reducing the area over which the energy of the mode-field is distributed, and (2) by increasing the nonlinear (intensity-dependent) index of refraction.
An article by N. Amitay et. al. entitled, "Optical Fiber Tapers--A Novel Approach to Self-Aligned Beam Expansion and Single-Mode Hardware" appears in Journal of Light Wave Technology, Vol. LT-5, No. 1, Jan. 1987. The article examines the behavior of single mode beam expansion during propagation through an optical fiber taper. The light transitions from a smaller to a larger mode-field diameter "adiabatically"; that is, in such a manner that conversion to higher order modes and excessive loss does not occur. The retention of a single-mode is important for the potential use of optical fiber tapers in fiber-to-fiber coupling; wherein, the expanded fiber core diameter at the large end of the taper relieves several alignment problems when mated with a similarly expanded fiber taper on the end of the mating fiber. The result should be a reduced degree-of-difficulty in achieving low-loss couplings. The experimental set-up shown in FIG. 10 of the referenced article utilized a conventional lens arrangement to focus the laser beam for injection into the cleaved end of a "standard" single-mode fiber. The beam expansion and output properties of an optical fiber taper at the far end of the fiber link were studied. The article by Dietrich Marcuse, entitled, "Mode Conversion in Optical Fibers with Monotonically Increasing Core Radius", appearing in Journal of Lightwave Technology, Vol. LT-5, No. 1, Jan. 1987, shows by mathematical modeling, the transition through a fiber taper (in the direction of small to large core radius) without suffering mode conversion to higher order modes (leading to single-mode light loss). The paper develops methods for accurately calculating light energy loss as a function of alignment parameters when expanded tapers are fused together.
The article entitled, "Tapered-Beam Expander for Single-Mode Optical-Fiber Gap Devices", in Electronics letters, 16 Jan. 1986, Vol. 22, No. 2, indicates ideas for application of tapered fibers, but concerns itself primarily with the beam expansion characteristics. An earlier writing in Applied Optics, Vol. 14, No. 12, Dec. 1975, entitled "Coupling Optical Waveguides by Tapers", by A. R. Nelson, also emphasizes the coupling of single-mode optical waveguides via expanding tapers. An investigation of the critical coupling tolerances associated with widening the guide by means of a taper uncovers the tradeoff of reduced tolerances for transverse displacement and tighter tolerances for angular alignment.
In Chapter 19 of the publication, Optical Waveguide Theory, A. W. Snyder and J. D. Love, Chapman and Hall, London, N.Y. (1983), there is a theoretical background for the concept of light concentration in the linear regime, and a certain condition called the "slowness criterion". While theoretical background for light concentration in the linear regime is presented in the above reference, no practical applications (i.e., to nonlinear optical devices) or methods for incorporation into actual (undersea or field deployable) fiber optic systems are discussed. In SPIE Vol. 460, Processing of Guided Wave Opto-electronic Materials(1984), "Laser-assisted Growth of Optical Quality Single Crystal Fibers", concerns only the production of perfectly cylindrically shaped single crystal fibers (constant radius versus length). The crystal materials are known to have much better thermal properties than silica glass and therefore, could stand a much higher energy density level of light at their narrow end. The article by Digonnet et al., appearing in the Journal of Light Wave Technology, Vol. LT-4, No. 4, Apr. 1986, pg. 454-60, entitled "1.064- and 1.32-micron Nd:YAG Single Crystal Fiber Lasers" is of interest with respect to single crystal, single-mode fiber lasers.
Thus, a continuing need exists in the state of the art for a simple device or method to provide "conditioning of the mode-field" of light energy propagation for nonlinear optical devices or circuits.